Some 27 miles east of London, Billericay was a market settlement situated atop a low ridge, oriented north-south, partly within the manor of Great Burghsted (now Burstead). Although the earliest known occurrence of Billericay's name is in 1291, as Byllyrica, the manorial lord, Stratford Langthorne Abbey, had in 1253 acquired, in the context of grant of various rights for the abbey estates, permission to hold within the manor of Great Burstead a Tuesday market and a fair around the festival of St. Mary Magdalene (July), which was the dedication of the parish church of Great Burstead. These grants were reconfirmed in 1285. In 1476 the abbot and convent were given a licence for a Wednesday market and an August fair at the festival of St. John Baptist (the dedication of the chapel mentioned below), while also reconfirming the July fair, all to be held at their town at Billericay. At an earlier period the choice of Wednesday for a market might have raised a complaint of damaging competition from nearby Brentwood; but not in the late fifteenth century. It may well be that the market and fair granted in 1253 were from the outset to be held at Billericay and a small market settlement may have been laid out there at that time, or at some point over the century that followed. The evidence for a planted town, however, is not strong, and it could be that urban character developed only over time.

A small Roman town seems once to have existed just south of the medieval settlement, on an east-west road crossing the ridge; this road later connected London and Rochford, and was crossed by a second, connecting East Anglia, via Chelmsford, to the Thames ferry at Tilbury – a half-mile stretch of this latter route later forming the High Street of Billericay; the crossroads lay a little west of the River Crouch. The site of Billericay proper, however, shows no signs of having been settled during the Roman or Saxon periods, although it has been under-investigated archaeologically. Perhaps scared off by the Roman cemetery there, Saxons settled two miles south at Burstead. Later, Billericay was established on what appears to have been marginal land, straddling the boundary between the parishes of Great Burstead, to the east, and Mountnessing, to the west; the dividing line may have been what became Billericay's High Street. Both parish churches were somewhat distant from Billericay; that of Great Burstead was held by Stratford Abbey and Mountnessing's by Thoby Priory. In 1303 the officials of the two churches were in dispute over ownership of the tithes and offerings of certain residents of the hamlet of Billericay. This problem may have been part of the reason why a chapel was built at Billericay in the early 1340s; later the part of Billericay within Mountnessing parish was transferred to Great Burstead parish, so that all its residents could use the chapel. It was rebuilt in brick in the fifteenth century; brick-making was then becoming a local industry, though several fifteenth-century Spanish tiles incorporated in the fabric hint at a wider trade in building materials.

At the time of Domesday, Burstead was in the hands of Odo, Bishop of Bayeux, but he forfeited his estates for rebellion against William Rufus. According to the abbey's attorney, defending a quo warranto challenge in 1285, Great Burstead later came into the hands of William Marshal (perhaps through his marriage to the heiress of the Earl of Pembroke in 1189), who granted it to Richard Siward, who in turn gave it to the abbey of Stratford Langthorne. This abbey had been founded 1135 by William de Montfichet and situated in the parish of West Ham, in a marshy area just north-east of London; it was absorbed into the Cistercian order in 1147. From modest beginnings, the abbey built up estates, mostly in Essex, whose exploitation made it wealthy; proximity to London must have helped it market the produce from those estates. The Cistercian order emphasized a working role for its monks, and they laboured to drain the marshes and extend their arable, as well as erecting workshops on the abbey site for industrial activities such as tanning, shearing, and weaving; they also operated several mills to produce flour used in baking bread that helped supply London's needs – the street leading to the abbey's main entrance gateway being later known as Baker's Row, while another entrance was known as Kilnhouse Gate. Before the close of the Middle Ages the abbey had its own small port on the Channelsea River.

Richard I confirmed in 1189 the abbey's lands and liberties, including the church of Great Burstead (built in the twelfth century), and granted the abbey permission to cut wood in the royal forest for fuel and building; at this time the abbey is documented as owning large flocks of sheep, meaning that it would have quantities of wool to sell. The thirteenth century saw the abbey channelling revenues into expanding its church, enlarging its precinct, and adding buildings therein. A tradition reported by Leland states that for a while the monks, chased out of their abbey by flooding, took up residence at Burstead, where the abbey had a grange, probably normally used for storing its wool prior to transport to English fairs or to the Thames for shipping abroad; although there is no corroborating evidence of this evacuation, it is plausible and may tie in with Richard II taking the abbey under his wing. The 1253 market grant lists the demesne lands of the abbey and shows it in possession of lands in Mountnessing, although it continued during the reign of Edward I to borrow money to buy up properties within that manor, which was principally in the hands of Thoby Priory.

All this set the scene for the establishment of Billericay, which contributed to Great Burstead being one of the most lucrative properties owned by the abbey, judging from the ecclesiastical taxation of 1291. What we know of the monastic community and the abbots' efforts to build the wealth of the abbey suggests that planting a market town at Billericay would have been a perfectly plausible initiative. The abbey is not known to have held a market or fairs at any other location.

It may well be that informal trade taking place at the crossroads mentioned above was what prompted the abbot at Stratford Langthorne to foster it, and benefit from it, by establishing a formal market. Furthermore if, as has been proposed [A Dictionary of British Place-Names, http://www.oxfordreference.com/view/10.1093/acref/9780199609086.001.0001/acref-9780199609086-e-1689?rskey=SCcldG&result=1701, last visited 9 February 2016], Billericay's name derived from the Latin bellerica, referring to a building for dyeing or tanning, industrial products could have been among the items traded at the crossroads; it is suspected that Tanfield Drive in present-day Billericay, located on the far side of what was Back Lane (see below), may encapsulate a memory of such activity. Since both industries produced pollution that would not be desirable close to a monastery, it is conceivable that any related facilities were required to be situated away from Stratford Abbey, and perhaps close to the river, which would supply the water needed for the industrial process (although this might have been obtained from springs known to have existed in the vicinity of the High Street).

Wool is another commodity likely available in some quantity for trade, judging from the number of sheep being raised at Great Burstead and in surrounding manors, according to Domesday Book. Wool continued to be important to the local economy in the fifteenth century. Several Colchester court rolls from the reign of Henry IV evidence Billericay merchants dealing in wool at the borough's market (presumably the dedicated market for wool there). In 1402, for example, six Billericay men – one named Benedict Wollemongere and another Richard Markaunt – were fined 12d. each for trying on repeated occasions to sell wool of sub-standard quality; it may perhaps have been the same Richard Markaunt, described as a husbandman, who was in 1429 being sued for debt by a London vintner. Another of that group, John Tannere, may have been the same man, described as a Billericay woolman, who was in 1423 suing a weaver in Maldon's court, and who in 1441, having bought up a thousand fleeces of wool in Kent, wished to ship it across the Thames at Tilbury (without having to pay toll to Port of London customs collectors) to sell to Colchester cloth-makers. Other wool traders from Billericay are mntioned in the Colchester rolls for 1405-07. And in 1491 two woolmen of Billericay claimed exemption from tolls at Colchester, on the grounds they were tenants of the king, specifically of his Duchy of Lancaster, presenting letters (newly drafted) sealed by the duchy's bailiff in Essex, stating that all residents of the duchy were exempt from toll throughout the kingdom. Upon their taking oath to the truth of the matter, their claim was allowed; we have no other indication the duchy held lands at Billericay, but perhaps the woolmen were tenants of duchy lands outside of their place of residence.

A licensed market at Billericay would in turn have attracted settlement around it, or possibly a planned settlement was established from the outset, adjacent to a market that had been moved slightly further north from the crossroads, to lay along the north-south street. It is thought that a market town grew up along that street – with the crossroads its southern terminus – in separate units on the east and west sides; though which side developed first is debated. A similar situation is encountered at Brentwood. Although the land on the west side was part of the manor of Cowbridge, this manor had come into the hands of Stratford Abbey, in part at its foundation. The author of the Extensive Urban Survey report, whose findings were shortly thereafter adopted for the county council report Essex Historic Towns: Supplementary Planning Guidance (1999), held that the western side saw the earliest development and that Billericay was "the type of medieval new town that grew in an organic manner due to the stimulus of trade, rather than because of official patronage" [Maria Medlycott, Billericay - Historic Towns Assessment Report, Essex County Council,1999, p.4, echoing Shirley Durgan, "Local government planning papers as sources for the local historian: an Essex survey", Essex Archaeology and History, ser.3, vol.19 (1988) p.233]. The eastern side of the High Street was assumed to be later infill of a marketplace located in a once-wider High Street. This conclusion is founded on the fact that the oldest building fabric still surviving is almost all on the western side of the High Street. However, as this represents a very small number of the Listed Buildings, with the oldest fabric dating only to the fifteenth century (though interiors of buildings have tended not to be included in surveys) – and one of the oldest being on Chapel Street, which was east of and converged with the High Street, and could have served as a back lane to properties on that side of the High Street – it is not a conclusion in which absolute confidence can be placed. The opposite side of the High Street was in a different parish until, following the erection of the chapel ca. 1345, transferred to Great Burstead parish. Most medieval buildings were replaced during the Tudor and Georgian periods, when Billericay reached a peak of prosperity before its market experienced a decline.

That tenements on the west side of the High Street are medieval is suggested by the fairly consistent street frontage of plots – approximating a perch in many cases – combined with a much greater depth (varying from 100 to 300 feet or more); also by the existence at the rear of many of these plots of a service route, long called Back Lane (now Western Road), although that it approaches at a tangent, rather than parallels, the High Street may mean it pre-existed plot allocation. There is thus some evidence for the possibility of a planned burghal unit along the west side of the High Street, without precluding organic growth in the form of subsequent expansion down the street. Along the east side of High Street old maps show far less consistency in plot size and shape; nor are tenements as deep, most coming to a halt at Chapel Street. If Chapel Street and Back Lane were ancient, albeit minor, tracks (perhaps just accessing surrounding fields, pasture, and woodland – that is, saleable materials – on the slopes of the ridge), then their points of convergence with the High Street could have been what suggested moving the marketplace northwards from the crossroads; the junction of Chapel Street and High Street was where the market cross and market house would be erected in the post-medieval period.

The marketplace stretched southwards from that junction in a triangular space that may have extended as far as the older crossroads market. The chapel (which of course required no cemetery) was built at the apex of the triangle, thus creating a precedent for later encroachment further south within the marketplace – Chapel Street looks narrowed at its northern end. Plots south of this triangle, on both sides of the street, are much less burgage-like, with wide frontages, and may represent later organic expansion in a context when there was little pressure for land. On the other hand, Chapel Street – or at least its northern section – may represent what remained of the east side of the marketplace after infilling at its northern end, originally in the form of two rows of shops. One of the first buildings facing onto Chapel Street, dated to late fifteenth or early sixteenth century, is believed to have been a merchant's house with a ground floor shop facing onto the street.

That Billericay was essentially a linear street settlement (along which post-medieval expansion continued southwards) is supported by the Great Burstead manorial court roll for 1495, which records the inhabitants of 'the street'; being presented en masse for failing to keep it cleared of dung. Yet, despite its modest size, there was something at Billericay able to attract settlers. This is instanced by a court roll of Ingatestone manor dating to the late 1390s: among a list of fugitive serfs compiled one was identified as having relocated in neighbouring Billericay, though most had removed farther afield; he was found and obliged to pay chevage for the right to continue living at Billericay, rather than return to his native manor. Billericay benefited in the Later Middle Ages by being on a route taken by Canterbury-bound pilgrims to the Thames, which they crossed at Tilbury; several inns appear to have existed there in the fifteenth century, and the building of the chantry may have been partly to cater to such a user group. We have little direct evidence of Billericay's commerce, however. A 1488 quitclaim had among its parties a barber and a butcher of Billericay, while a metal-worker (surnamed Plomer, alias Ledder) and an unnamed weaver were among several Billericay supporters of the Peasants' Revolt (which had Stratford Abbey as one of its targets for looting and burning of records).

On the basis of what little evidence we have, it is hard to assign even an approximate date to when Billericay may be considered to have become urban, in character if not in name. If we judge that trade and industry were associated with Billericay from its earliest appearance, then the acquisition of a market licence, and the appearance of a block of burgage-type plots along the west side of the High Street might well be indicative of a planted market settlement, and we should assign urbanization to the late thirteenth or early fourteenth century. Yet if we see organic development from a hamlet with market into something larger and more diversified both in terms of occupations and wealth levels, the evidence points rather to the late fourteenth and fifteenth centuries. The lack of any reference to burgesses or burgages during the Middle Ages leaves us hanging. In a message from the king to the Essex escheator in 1385, Billericay could still be perceived as a secondary settlement within the vill of Great Burstead, but this is not a source on whose perceptions we should rely too much, particularly given the reluctance of ecclesiastical seigneurs to accord borough status to their market settlements. At least we need not doubt the predominantly commercial character of Billericay during the Late Middle Ages; though today it has spread out to become essentially a suburban dormitory settlement, the historic core still retains its character as a centre for shopping focused on an axial street.